Current medical imaging systems use a klystron to develop X-rays for medical therapeutic use by impinging a high speed electron beam onto a target which generates x-rays, and the x-rays are used for treatment of cancerous tumors. In a clinical use linear accelerator (such as the CLINAC® system manufactured by Varian medical systems), a klystron, linear accelerator, and x-ray target are mounted in a gantry that rotates around a cancer patient receiving radiation therapy, with the X-rays directed into a target tumor with high precision.
A typical medical klystron requires on the order of 50 KW of power, roughly half of which is used to energize a solenoidal coil which generates the main axial magnetic field. The resulting overall size and power consumption of the main axial field components results in a system which requires special siting considerations.
For large accelerator systems, elimination of the solenoid and the associated power supply and cooling circuitry also impacts the operating cost. For large klystrons, the solenoid coil can require 20 kW or more. In addition, water cooling is required to remove power generated by resistive losses in the coils.
While operating costs are important for clinical linear accelerator klystrons, equally important considerations are size and weight. The klystron and associated power supplies and cooling are mounted in the gantry, and are significant contributions to the size and stresses on the structure, and accordingly, the large size requirements of the prior art klystron exclude potential installations due to size considerations. Reliability is also an important consideration. Replacement of the solenoidal coil, and associated power supply and cooling system with permanent magnets removes several potential failure modes.
Compared to prior art traveling wave tubes (TWT), klystrons have greater efficiency (typically two to three times greater than TWT). However, the klystron also has specific requirements that cause difficulty in design and implementation. Whereas a TWT tends to have an electron velocity at the final RF output which varies only slightly from maximum to minimum velocity, the final electron velocity in the output cavity of a klystron has a much greater variance, including the possibility that the electron velocity in the klystron may approach 0, which can cause retrograde electron movement, causing an associated degradation in efficiency. In a helical TWT, no RF cavities are present, and in a coupled-cavity TWT, the sequences of cavities are very uniform and confined to within the PPM magnet structure. Consequently, the circuit structure in a TWT does not impact the geometry of the magnet circuit. In a klystron, the RF cavities along the axis are placed with irregular periodicity according to the resultant beam characteristics, and as a result, the circuit structures and the PPM structures must be integrated, since they overlap each other radially.
Klystrons typically have an efficiency that is two to three times greater than a TWT, and because of this efficiency, as well as the difficulty in cooling the helical wave structure of a TWT, a high power klystron will often operate at a much higher power level than a high power TWT. Consequently, there are requirements for increased cooling of the circuit regions of the klystron, and unlike TWT circuits, direct cooling of the klystron RF circuit is required. Moreover, klystrons use resonant cavities to bunch and extract energy from the electron beam, and precise frequency control of the individual cavities is required. This may be accomplished using mechanical structures to tune the RF cavities to the correct frequencies. This is not required in TWTs, since they do not use resonant structures.
It is desired to provide a klystron with cooling for the RF cavities and access to the RF cavities for frequency tuning structures, and optionally to provide cooling for the beam tunnel structures, if required. It is further desired to provide a klystron for a therapeutic treatment system with reduced size, elimination of the requirement for an electromagnetic axial field generator and associated cooling requirement, and which provides for high power operation.